(742d) Industrial-Scale Modification of Powders By ALD Technique

Atomic layer deposition (ALD) is a gas-phase method generally used for the deposition of high-quality thin films on various material surfaces. The characteristics of ALD is the deposition of well-controlled structures on both flat and 3D –surfaces in a layer-by-layer manner at relatively low temperatures. During ALD processes all surfaces of substrates are coated in a uniform manner.

In the field of catalysis, ALD can be used to tailor new functionalities and surface structures on the surface of powders without the disturbance of organic solvents. For example, platinum group metals can be deposited on the catalyst support with excellent dispersion without the disturbance of organic solvents. ALD enables also bimetallic modification of the catalyst support surface. The ALD -type growth enables the control of particle size of the metal nanoparticles to be deposited on the surface by altering the number of ALD cycles applied. Additionally, deposition of continuous thin films on powders enables change and control of the physical and chemical properties of powders by passivation of material and increase of stability against oxygen, moisture or corroding chemical substances, for example. ALD has also been used to glue and increase the adhesion of particles prepared with other techniques on planar substrates.

In many industrial applications, large amounts of powders need to be modified. However, there are challenges in up-scaling of ALD powder coating reactor because of resistance of gas flow through the powder bed and the sample holder. Here we present various kinds of reactor constructions for the modification of powders in order to achieve uniform films or excellent dispersion of metals on the powder surface. The main powder coating construction is based on down-flow design shown in Fig. 1 (left) where also a graphical example of simulated reactor pressure is shown. Simulation tools for scaling-up of powder depositions has been successfully applied to help the scale-up of ALD powder coating reactor. In this work simulation of carrier gas flow through the ALD powder coating reactor was done in order to analyze how ALD reactor dimensions, like the filter diameter, affect the pressure.¹ The results can also be used to estimate the maximum amount of powder that can be coated under desired conditions in the reactor construction.

This work is partially funded by 7^th European Framework Programme 4G-PHOTOCAT (309636).

Fig. 1. Left: Cross section of simulated reactor pressure with 48 mm aluminum oxide powder bed. Right: Measured and simulated pressure drop over 48 mm high Al₂O₃ powder bed (of grain size 63-90 µm).

References:

1. Vähä-Ojala, T, Master’s thesis, Aalto University, 2014-12-15, 66 p.